All-frequency generator



June 23, 1942.

A. M. WENGEL .Y

ALLJREQUENCY GENERATOR Filed .June 17, 1940 2 sneetssneet 1 June 23, 1942. v A M WENGEL v2,287,401

ALL-FREQUNCY GENERATOR Filed June 1v. 1940 2 sheets-smet 2 l l l 1 Patented June 23, 1942 UNITED STATE ALL-FREQUENCY GENERA'roR Arthur M. Wengel, Madison, Wis., assignor to v Ray-O-Vac Company, a corporation of Wisconsin Application June 17, 1940, Serial No. 341,015

s claims. (ci. 179-1) This invention relates to an all-frequency generator for producing a wave output comprising a substantially uniform distribution of frequencies throughout a desired band, and more particularly to a generator for test purposes, as for producing a background sound for use with a separate substantially single frequency test note for hearing testing purposes.

lar, and the conditions of use normally encountered by a hard-of-hearing person; yet another feature of this invention is that testing of ear response to a particular test note is done under laboratory conditions which closely approximate conditions of actual normal use of a hearing aid device; a further feature of this invention is that, at least insofar as ear response is concerned, it provides abackgroundjsound comprising a,l substantially uniform distribution of frequencies, of uniform intensity, throughout a given desired audiblel band; another feature of this invention is that the width and location of the background sound band, as well as its intensity, are readily controllable; still another feature of this invention is that new and improved means are provided for varying the resonance frequency of a tuned clrcuitwithout mechanical movement of any of the elements thereof; other features and advantages of this invention will be apparent from the following specification and the drawings,inwhich:

Figure 1 is a block diagram of apparatus embodying my invention; Figure 2 is a wiring diagram of apparatus embodying this invention;

and Figure 3 is a wiring diagram of another embodiment of my invention.

It is known that the hearing response of the ear of ajha-rd-of-hearing person differs for pure notes of various frequencies, and some eiort has l heretofore been made to design the amplifier of .a'hearing aid device to have a frequency-amplication curve inverse to theA frequency-response curve of the ear. That is, the amplifier is designed to accentuate those frequencies to which the ear is least responsive andfto amplify to a lesser extent those frequencies to which the ear is particularly responsive. A hearing aid device having an amplifier of this type has been disclosed in my earlier Patent 2,192,669, which issued March 5, 1940.

Heretofore, however. audiometers or test apsponse curve of the ear of a hard-of-hearing person have all been capable of determining only the threshold curve or the pain curve. That is, such previously used test apparatus has had means for lproducing notes of any desired frequency in the audio range and means for varying the intensity of such ``notes, generally combined with a meter or the like for measuring such intensity. With such apparatus, at any given frequency, the lowest (threshold) intensity audible to the person could be determined and the intensity which produced pain or discomfort. This enabled plotting of the two curves representing the extremes of hearing response; but gave no information as to the intermediate areabounded by such curves. l

The threshold and pain curves seldom, if ever, have the same form; and I have found that hear-v ing response at intermediate points may differ from the shape of both curves. In the normal aifairs of every day life it is the intermediate area, and not the threshold curve, which is important. That is, the ears of a person are prac` tically always exposed to a background of sound comprising practically all frequencies; and sounds which it is desired tok hear must make themselves audible above such background. A person sitting in an ollice in the business area of a city, for example, will have his ears exposed to a background of noise produced by street cars, trains, motor traiflc, typewriters, telephone conversations at other desks and a number of'other sources. Under such circumstances the threshold audibility curve determined `in a laboratory under sound insulated conditions is of little value in designing a hearing aid device.

My present invention comprises apparatus for electrically producing a complex electricaliwave or waves having a substantially uniform distribution of frequencies, of the same intensity, throughout a given band in the audible range, the complex wave being controllable in respect to intensity level, band width, and band location. When used for hearingaid work this Wave is translated into a corresponding background sound, and this provides improved apparatus for use with an audiometer producing a substantially single frequency test note controllable as to frequency and intensity, preferably by calibrated dials, in order to design the frequency-amplification curve of a hearing aid ampliner with proper corrections for vthe ear response under normal usage conditions, rather than threshold or pain level conditions. An audiometer satisfactory for such use with the all-frequency generator disclosed herein is shown, for example, in the lower half of Figure 1 of my application, Serial No. 254,931, mentioned above. i

In the particular embodiment of my invention paratus used to determine the frequency-re- 6o illustrated herewith in Figures l and 2, referhundred kilocycles.

ring first to Figure l, a fixed oscillator generates a single fixed frequency, as for example one The variable oscillator is adapted to have the frequency generated by it varied through a band close to the frequency generated by the fixed oscillator, as for example through a band of ninety to one hundred kilocycles'. This is accomplished electrically by frequency-controi means which is put through a rapid periodic cycle, as for example two hundred or five hundred or one thousand times per second, by a sweep oscillator. The outputs of the fixed and variable oscillators are combined to produce a beat frequency wave or note which is then passed through an amplifier which-preferably includes a low-pass filter to remove the higher frequencies and leave only the beat note frequencies. y

This beat note frequency goes through the desired band the desired number of times per second, so that the ear hears only a background sound comprising a substantially uniform distribution of frequencies of the same intensity.

That is, if the variable oscillator swings from ninety to one hundred kilocycles five hundred times a second, a few waves of each of the frequencies from substantially zero frequencies up to ten thousand cycles per second would be generated five hundred times a second, so that the ear would not be able to distinguish, but would merely hear the desired al1-frequency background. The sweep oscillator frequency can be made below normal audibility, if desired, or can be filtered out of the output by a sharp single frequency rejection tuned circuit in the amplifier.

The resultant background sound output is combined with a substantiallysingle-frequency test note generated by an audiometer, and the combined electrical waves are translated into actual sound waves by an earphone, and thus delivered to the defective ear being tested. If calibrated frequency intensity and band width control dials are used, as is preferable, intensities and frequencies of all desired types can be applied to the ear for test purposes to determine the best amplifier prescription for that particular ear in order to enable it to hear best under normal every-day conditions.

Referring now more particularly to Figure 2, the fixed oscillator is shown as comprising the tube I with its associated tuned circuit (the inductance II and condenser I2) to determine its frequency of oscillation. The circuit constants may be so chosen, for example, as to have this oscillator generate a frequency of one hundred kilocycles per second.

It is here shown as resistance coupled to the first tube I3 of an amplifier comprising the tubes I3, I4 `and I5. lThese various tubes of the amplifier are also resistance coupled, a low-pass filter I5 being in the circuit path between two of the tubes. The output of the amplifier is coupled, through appropriate transformers and a calibrated T-pad or resistance network, to the jacks I1 and I8. The jack I'I is intended to have the output of an audiometer plugged into it; and the jack I8 is intended to have an earphone plugged into it, s0 that the earphone delivers 'to the ear being tested both the output of the amplier and the substantially single frequency test note from the audiometer. A portion of the output of the amplifier may be diverted through the wire I3 to an electronic tuning eye, milliammeter, or other means for determining zero beat conditions.

'I'he input of the amplifier also has delivered to it, through a wire 20 connected to the grid circuit of the ltube I3, the wavesgenerated by a variable oscillator comprising the tube 2| I and its associated circuits. Thetuned circuit associated with the tube 2l to determine the frequency ai: which it oscillates includes the inyductance 22 and a plurality of parallel condensers.

These condensers are the fixed condenser 23, the main variable condenser 24, and the zero beat adjustment condenser 25. The values of the tuned circuit constants are so chosen that with the condenser 24 at minimum capacity the condenser 25 can be adjusted to make the frequency generated by the tube 2I exactly equal to that generated by the fixed oscillator tube Il), so that it will be zero beats. The condenser. is preferably calibrated in the cycles of the lbeat note resulting from its adjustment, so that movement of this condenser to a particular position determines 'the ,low frequency boundary of the sound band which is to be generated.

`A lead 26` and condenser 21 couplel the grid end of the inductance 22 to the plate of a control tube 28. This tube has at least three elements, and-is preferably a multielement; tube, as shown. Its control grid 29 is coupled through a resistor 30 and a. condenser 3I to its plate; and the control grid also has impressed thereon a sweep frequency voltage, as from an audio or radio frequency oscillator 32 i of conventional type. 'I'he sweep' voltage from this source is impressed across a potentiometer 33, and any desired portion of it delivered to the control grid 23 through a resistor 34. Adjustment of the variable arm on the potentiometer 33 determines the maximum amount of sweep voltage to be impressed on the grid, and` thus the vmaximum swing frequency generated by the tube 2I. That is, the lower boundary of the all-frequency band is determined by adjustment of the condenser 24; and its width by adjustment of the sliding arm of the potentiometer 33.

The purpose of the tube 28 is to provide a readily electrically variable impedance in a control circuit having the current flow therein out of phase with the voltage in the tuned circuit of the oscillator tube 2I tube parallels that through the inductance '22; and if it lags the voltage in the tuned circuit of the tube 2I, it acts as an additional effective inductance in such tuned circuit. The shift in phase relation is generally sufficiently effected by the capacity between tube elements and wiring, here indicated in dotted lines as the capacity 35. This should be between fifteen and fifty micro-microfarads where the tubes 2| and 28 are of the 6J? type; and must be low.in impedance compared to the resistor 30.

Assuming the condenser 24 to be so set that the lower margin of the band, or the beat note produced in the amplifier output when the sweep voltage is zero, was one thousand cycles per second, rise in sweep voltage` would increase current flow through the tube 23, and thus increase the effective inductancefin the tuned circuit of the variable oscillator 2I. This'would in turn decrease the frequency generated by it, so that it would drop on down from ninety-nine kilocycles to ninety-six kilocycles, for example, as the sweep Avoltage rose to maximum. The beat note frequency generated by the combination of the two oscillators wouldthus swing through all of the frequencies in the range from one thousand to four thousand cycles in the length of time re- Current flow through this ydenser 21.

ohms as against fifty ohms.

quired for the sweep voltage to rise from zero to maximum; and the intensity of all of these Waves would be substantially the same, and their distribution with respect to time also substantially uniform. If the sweep'frequency were fifty cycles per second, each of the various frequencies throughout the desired band would be generated forr a brief instant fifty times per second; and the resultant effect on the ear would be that of a background sound comprising a substantially uniform distribution of frequencies, of uniform intensity, in the audible band between one thousand and four thousand cycles.

While the majority of the circuit constants follow conventional amplifier and oscillator practice, it is believed desirable to mention specifically a few of the others. Oscillator condensers 31, for example, should be about .1 mf.; and condensers 38 should be .01 mf. Oscillator coupling condensers 39 may be .0005 mf., as may also con- Each of the three resistors 40 of the T-pad network may be five hundred ohmsrthe resistor lil should be a number of times the value of the resistor 42, as for example two thousand The potentiometer 33 preferably has a resistance in the neighborhood of fifty thousand ohms, and that of the coupling resistor 34'in the neighborhood of 250,- 000 ohms. The condenser 3i usedby me has a capacity of .0002 mf.; and the resistor 30 a. value of fifty thousand ohms. The inductance of the y choke 43 should have a value of at least 85 millihenries, preferably higher.

Another embodiment of my invention is illustrated herewith in, Figure 3. 'Ihe tube 50 and its associated tun/ed circuits form the fixed oscillator; the tubes jl, 52 and 53 comprise the amplifier, the output again being delivered through a T-pad to jacks adapted to be connected to a source of a single frequency note (the jack 54) and to a pair of earphones, through the jacks 55 and 56, provision beingy made to connect the phones in parallel or in series. 'I'he tube 51 and its associated circuits comprise the variable oscillator; the tube 58 is the control tube for varying the flow of out of phase current drawn from one of the'oscillators, in this case the so-called fixed oscillator; the tube 59 and its associated circuits form the sweep oscillator, in this case the tube being of the 884 type, and generating a sawtooth wave with a lineal change of voltage; and

the tube 60 is merely a simplifying -tube connecting the sweep oscillator to the control tube.

Instead of being a continuously variable control, the sweep frequency oscillator is here arranged with step-by-step control, means being provided for switching any of a plurality of condensers into the circuit by switch means 6|. The capacitanceof the fixed condensers is preferably so vchosen as to provide a sweep frequency of five, ten, fifteen, thirty, sixty and one hundred twenty cycles per second. Control of the sweep width is again provided by a potentiometer, here identified as 62. In this embodiment of my invention it is the oscillator provided with a fixedly tuned circuit which is swung by the control tube; and the variably tunable oscillator, including the tube 51, remains at the frequency to which it has been set. The resultant beat note swing is, of course, the same as in the previously described modification. The use of a sawtooth generator provides a lineal change of voltage and an exceedingly uniform distribution of frequencies in the desired output wave.

embodiments of my invention it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the. appended claims.

I claim:

1. Apparatus of the character described for producing, for hearing testing purposes, a background sound comprising a substantially uniform distribution of frequencies and a separate singlefrequency test note. including: an oscillator having a tuned circuit determining the frequency generated; means for rapidly periodically varying the effective inductance in the tuned circuit through a predetermined range; means for generating a substantiallyl single-frequency note; and means for simultaneously impressing the background sound and the note on the ear being tested.

2. Apparatus of the character described for producing, for hearing testing purposes, a background sound comprising a substantially uniform distribution of frequencies and a separate singlefrequency test note, including: an oscillator having a tuned circuit determining the frequency generated; a control circuit operatively connected with the tuned circuit and arranged to have variations in the flow of current therethrough effect variations in the effective reactance of the tuned circuit; means for rapidly periodically varying the impedance of the control circuit to vary the frequency generated; means for generating a substantially single-frequency note; and means for simultaneously impressing the jbackground sound' and the note on the ear being tested.

3. Apparatus of the character described for producing, for hearing testing purposes, a background sound comprising a substantially uniform distribution of frequencies and a separate singlefrequency test note, including: a first oscillator having a tuned circuit determining the frequency generated; a second oscillator having a tuned circuit determining the frequency generated; means for combining the outputs of the two oscillators to produce beat frequency waves within the audible range; a control circuit in parallel with the tuned circuit of the second oscillator and arranged to have variations in the iiow of current therethrough effect variations in the effective reactance of the tuned circuit it parallels; means for rapidly periodically varying the impe, dance of the control circuit to vary the frequency generated by the second oscillator, and the beat frequency; means for generating a substantially single-frequency note; and means for simultaneously impressing the background sound and the note on the ear being tested.

4. Apparatus of the character claimed in claim 3,v including means for predetermining one boundary of the desired band.

5. Apparatus of the character claimed in claim 3, including meansfor predetermining the widt of the desired band,

6. Apparatus of the character claimed in claim 3, wherein the control circuit includes a vacuum tube having at least three elements therein and the cathode-plate impedance of the tube is varied by periodically varying the voltage on the grid of the tube.

ARTHUR M. WENGEL.

While I have described and claimed certain 

